1. Field of the Invention
The invention relates to the field of production of micrometer- and sub-micrometer-sized monodisperse or narrowly-sized droplets and transport of the liquid feed. Monodisperse or narrowly-sized droplets less than 10 μm in diameter in sprays are highly desirable in nanoparticles synthesis from heat sensitive precursor solutions because they can be processed at a conveniently low temperature and atmospheric pressure. Also, monodisperse or narrowly-sized droplets (particles) 1 to 6 μm in diameter have multiple biomedical applications including pulmonary drug delivery, micro encapsulation of drugs, and drug preparation for inhalation. Other potential applications include thin-film coating and three-dimensional (3-D) spray coating for micro- and nano-electronics and photonics.
2. Description of the Prior Art
Droplet (or drop) formation based on acoustic techniques has been an important research and development subject area in recent years because of its various potential applications. For example, nozzle-less droplet ejectors that use focused acoustic waves at 5-900 megahertz (MHz) to eject micrometer-sized droplets, one drop at a time, are applicable to high-resolution ink-jet printing. A micromachined ultrasonic droplet generator based on a liquid horn structure is capable of droplet ejection from 5- to 10-μm orifices at multiple resonant frequencies between 1 and 5 MHz and has been shown to be applicable to inhalation drug therapy. Piezoelectrically actuated flextensional micromachined ultrasonic transducers with annular piezoelectric disk and operating resonance frequencies from 450 kHz to 4.5 MHz have also been designed and fabricated for droplet ejection. Current ultrasonic nebulizers that use vibrating micrometer-sized mesh technology are also capable of producing micrometer-sized droplets for applications such as pulmonary drug delivery, but the resulting drop size distributions are rather broad because various atomization mechanisms such as cavitation, impinging, and jetting are also involved in addition to the capillary wave mechanism. Furthermore, the electrical drive power required is typically greater than 10 W. Finally, commercially available metal-based bulk-type ultrasonic nozzles (Sono-Tek Corp., Milton, N.Y.) that use capillary waves at frequencies far below 1 MHz are capable of producing sprays of droplets (mist) with a diameter smaller than the nozzle orifice diameter, but much larger than 10 μm. All of the aforementioned devices under research and development and commercial devices are either incapable of producing monodisperse or narrowly-sized droplets of such desirable size range (1 to 6 μm) or producing them only at very low throughput. Furthermore, they are bulky, requiring much higher electrical drive power, and do not have potential for mass production using batch fabrication. Clearly, it is desirable to explore and realize miniaturized ultrasonic nozzle devices that operate at megahertz drive frequencies for producing sprays of micrometer-sized or even sub-micrometer-sized droplets with narrow size distributions, or even monodisperse droplets, at high throughput and low electrical drive power.
Additionally, several attempts have been made in the prior art to transport the liquid to be atomized through the ultrasonic nozzle in which it is to be ejected from. Typically, a central channel was used in the prior art to transport the liquid to be atomized to the endface of the nozzle tip of the single-nozzle ultrasonic device. Specifically, in the Sono-Tek metal-based bulk-type ultrasonic nozzle referred to earlier a cylindrical channel is bored through the nozzle body. In the silicon-based multiple Fourier horns ultrasonic device a pair of identical nozzles each with an etched trough along the nozzle axis was bonded to form a 200 μm×200 μm central channel for transport of the liquid to the endface of the vibrating nozzle tip. As the amplitude of vibration on the endface of the nozzle tip in the direction perpendicular or nearly perpendicular to the endface reaches a threshold or critical vibration amplitude at the MHz resonance frequency of the multiple Fourier horns, the liquid resting on the endface is broken into micrometer-sized monodisperse or narrowly-sized droplets. Formation of this central channel requires several additional micro-fabrication steps. Furthermore, as the operating frequency of the nozzle increases, the physical size of the device decreases. As a result, the area of the vibrating endface decreases, and the degree of complexity involved in channel fabrication increases. Finally, the corresponding reduction in the channel cross section will result in decreased liquid flow rate and, thus, droplet throughput.
What is needed is a method and device or devices that eliminate the need for a central channel and its corresponding fabrication complexities while at the same time maintaining a high flow rate and a high-throughput production of monodisperse or narrowly-sized droplets in micrometer and sub-micrometer sizes, and low electrical drive power.